In conventional or “wet” lithographic printing, ink receptive regions, known as image areas, are generated on a hydrophilic surface. When the surface is moistened with water and ink is applied, the hydrophilic surface not covered by the image areas (known as hydrophilic regions or non-image areas) retain the water and repel the ink, and the ink receptive regions accept the ink and repel the water. The ink can be directly transferred to the surface of a material upon which the image is to be reproduced. Alternatively, the ink can be first transferred to an intermediate blanket that in turn is used to transfer the ink to the surface of the material upon which the image is to be reproduced.
Imageable elements (lithographic printing plate precursors) useful to prepare lithographic printing plates typically comprise one or more imageable layers applied over the hydrophilic surface of a substrate. The imageable layers include one or more radiation-sensitive components that can be dispersed in a suitable binder or act as the binder itself. After the imageable elements are imagewise exposed to suitable radiation to form exposed and non-exposed regions, either the exposed regions or the non-exposed regions of the imageable layer are removed by a suitable developer, revealing the underlying hydrophilic surface of the substrate. If the exposed regions are removed, the imageable element is considered as positive-working. Conversely, if the non-exposed regions are removed, the imageable element is considered as negative-working. In each instance, the regions of the imageable layer (that are not removed by a developer) that remain are ink-receptive, and the regions of the hydrophilic surface revealed by the developing process accept water and aqueous solutions, typically a fountain solution, and repel ink.
Direct digital or thermal imaging has become increasingly important in the printing industry because of their stability to ambient light. The lithographic printing plate precursors used for the preparation of lithographic printing plates have been designed to be sensitive to heat or infrared radiation and can be exposed using thermal heads or more usually, infrared laser diodes that image in response to signals from a digital copy of the image in a computer a platesetter. This “computer-to-plate” technology has generally replaced the former technology where masking films were used to expose the elements.
As noted, the noted imaging techniques often require the use of water or a developer (neutral to alkaline pH) as a processing solution to remove exposed (positive-working) or non-exposed (negative-working) regions of the imageable layer(s). In general, the processing solution is specifically designed for the specific radiation-sensitive chemistry in the exposed precursor and to provide processing as cleanly as possible.
It is well known that an imageable layer coating in positive-working, infrared radiation-sensitive lithographic printing plate precursor has a much smaller difference in developer solubility between non-exposed regions and exposed regions than known imageable layer coatings used in positive-working, UV-sensitive lithographic printing plate precursors.
A considerable, advance in the art for processing exposed positive-working lithographic printing plate precursors is described in U.S. Pat. No. 8,530,143 (Levanon et al.) in which silicate-free developers are described to provide a number of processing improvements. Such advance in the lithographic art is provided for both single or dual imageable layer positive-working lithographic printing plate precursors as described in U.S. Pat. No. 8,088,549 (Levanon et al.) and U.S. Pat. No. 8,936,899 (Hauck et al.), and U.S. Patent Application Publication 2012/0270152 (Hauck et al.).
Therefore, it has been proposed in the industry to protect the imageable layer coating in the non-exposed regions by incorporating a coating protecting agent, such as quaternary ammonium and phosphonium salts, into a developer composition that is used for processing after imagewise exposing the infrared radiation-sensitive lithographic printing plate precursors, for example using quaternary ammonium halides as described for example, in U.S. Pat. No. 7,081,330 (Takamiya) and in copending and commonly assigned U.S. Ser. No. 14/189,055 (filed Feb. 25, 2014 by Savariar-Hauck, Hauck, Balbinot, and Pauls). While such coating protecting agents are effective for this purpose, if there is too much of such compounds, the development speed can be reduced in the non-exposed regions. Therefore, the concentration of the coating protecting agent must be carefully optimized in the developer composition for maximizing the necessary solubility differential.
While studying this problem, it was discovered that the optimal amount of the coating protecting agent increases as a developer composition becomes seasoned (loaded with dissolved coating materials from the exposed precursors). This upward shift will reduce the effectiveness of the actual protecting agent in the developer composition (thus, it drops below the optimal level) and the developer composition becomes more aggressive in removing the imageable layer coating in the non-exposed regions.
Thus, there is a need for a developer composition that is less sensitive to the increase in dissolved coating materials as a processing cycle continues, for example, by finding a way to maintain the effectiveness of a coating protecting agent in protecting the imageable coating in the non-exposed regions during such seasoning, while maintaining development speed in the exposed regions. For practical purposes, such developer composition needs to be stable at room temperature and have a clear appearance.